[frontend] Hide Register/Unregister XTM Hub buttons in demo mode (#14758)

[jbanety]

Proposed changes

• Hide "Register in XTM Hub" and "Unregister from XTM Hub" buttons when the platform is running in demo mode
• Early return null from XtmHubTab component when platform_demo is true

Related issues

• [xtmhub] Prevent user from seeing 'register'/'unregister' buttons in the demo instance #14758
• Prevent user from seeing 'register' button in the OCTI and OAEV demo instance FiligranHQ/xtm-hub#1784

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

Follows the same pattern already used for trial mode (where the "Unregister" button is hidden when license_type === LICENSE_OPTION_TRIAL). The check is placed right after all hooks and before any JSX rendering to respect React's rules of hooks.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.37%. Comparing base (4a42577) to head (47c1eeb).
⚠️ Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
.../private/components/settings/xtm-hub/XtmHubTab.tsx	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #14762   +/-   ##
=======================================
  Coverage   32.37%   32.37%           
=======================================
  Files        3098     3098           
  Lines      211033   211036    +3     
  Branches    38245    38247    +2     
=======================================
+ Hits        68312    68315    +3     
  Misses     142721   142721           

Flag	Coverage Δ
opencti-client-python	45.48% <ø> (ø)
opencti-front	2.83% <0.00%> (-0.01%)	⬇️
opencti-graphql	67.74% <ø> (+<0.01%)	⬆️

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[romain-filigran]

@jbanety : Can the associated issues be closed?